1. Field of the Invention
The present invention relates to a process for producing an aromatic polycarbonate, and more particularly, to a process for producing an aromatic polycarbonate having a controlled molecular weight, a high rate of end capping and less-coloration.
2. Description of the Related Art
Aromatic polycarbonates are engineering plastics having transparency comparable to glass, outstanding impact resistance, excellent heat resistance, dimensional stability, and weatherability in combination.
Further, with respect to the problems of recycling of plastics which are recently discussed, aromatic polycarbonates are thermoplastic polymers and the recycling is easy, and therefore, aromatic polycarbonates are widely used in the field of motor cars, electric machines and the like.
However, in the case of aromatic polycarbonates produced according to conventional methods [for example, feeding phosgene to a two-phase mixture of an aqueous alkali metal salt solution of an aromatic dihydroxy compound in the presence of a monohydric phenol such as p-tert-butylphenol as a molecular weight controlling agent to form an aromatic polycarbonate oligomer having a chloroformate group of a low molecular weight and then polymerizing said oligomer, that is, an interfacial polymerization (cf. H. Schnell, "Chemistry and Physics of Polycarbonate", Interscience Publishing, p. 33-41 (1964))], only about 70-80 mole % of the total end groups of the aromatic polycarbonates is capped, and in particular, there is a drawback that the rate of end-capping of the oligomer region is low.
It is known that in the case of such aromatic polycarbonates having a low rate of end capping there occurs rearrangement of the polymer chain when melted and thereby the molecular weight and the molecular weight destribution change [Kobunshi Kagaku, 21, 505 (1964), J. Polymer Sci., 55, 251 (1961)].
It is known to those skilled in the art that molecular weight and molecular weight distribution most closely relate to physical properties of polymers, and when the above-mentioned phenomena occur, it is clear that physical properties of aromatic polycarbonates are changed or lowered by repeating melt-molding for recycling aromatic polycarbonates.
Therefore, there has been a demand to develop a process for producing an aromatic polycarbonate which has a very high rate of end capping and is difficult to subject to rearrangement of the polymer chain.
In the above-mentioned production method, a method for producing an aromatic polycarbonate is known which comprises adding a polymerization catalyst (also called "polycondensation catalyst") such as a tertiary amine and the like so as to accelerate the polymerization of oligomers (for example, U.S. Pat. No. 3,275,601). According to this method, the addition of the polymerization catalyst is effected after the reaction of a carbonyl halide compound with an aromatic dihydroxy compound is completed and oligomers of the aromatic polycarbonate are formed. However, it is difficult for this method to produce aromatic polycarbonates having a high rate of end capping.
It is also known that a haloformation reaction is carried out in the presence of a polymerization catalyst by adding the polymerization catalyst to the first step of the production of aromatic polycarbonates, that is, a haloformation reaction.
When at the beginning of the reaction there are present the total amounts of the materials necessary for producing an aromatic polycarbonate, that is, a polymerization catalyst, a molecular weight controlling agent, and an alkali metal or alkaline earth metal base in the reaction system, it is difficult to control properly the molecular weight of the aromatic polycarbonate and the resulting molecular weight distribution becomes very wide.
For the purpose of controlling molecular weight properly, for example, U.S. Pat. No. 3,240,756 proposes a method that a molecular weight controlling agent is fed as a carbonyl halide is fed in the case of reacting an aromatic dihydroxy compound with a carbonyl halide compound in the presence of a polymerization catalyst.
However, U.S. Pat. No. 3,240,756 discloses working examples only for using quaternary ammonium salts as the catalyst, and when the present inventors traced said polymerization method by using a tertiary amine, it was very difficult to control the molecular weight. U.S. Pat. No. 3,173,891 discloses that in a process for reacting an aromatic dihydroxy compound with phosgene and an alkali metal hydroxide in an amount sufficient to impart to the reaction system a pH value of 10.5-11.8, there are present a polymerization catalyst and a molecular weight controlling agent to produce an aromatic polycarbonate.
However, according to this process, an aromatic polycarbonate having a controlled molecular weight is obtained, but the polydispersity index (ratio of weight average molecular weight to number average molecular weight) is high and the resulting aromatic polycarbonate has poor heat decomposition resistance and impact resistance.
This is also disclosed in U.S. Pat. No. 3,989,672. This U.S. Patent discloses a process for producing a polycarbonate polymer having improved resistance to thermal degradation and impact resistance consisting of reacting a carbonate precursor with 2,2-bis(4'-hydroxyphenyl) propane ["Bisphenol A"] in an aqueous medium containing sufficient alkali metal hydroxide to maintain a pH between 8.0-10.2 during the reaction. In the embodiment there is described that the molecular weight controlling agent and the polymerization catalyst are present in advance.
According to this process, the rate of end capping of an aromatic polycarbonate can be improved to some extent, but the resulting rate of end capping is not always satisfactory.
The present inventors have traced the reaction of this process where a pH is 8.0-10.2 and found that the reaction starts in such a state that the aromatic dihydroxy compound is not completely dissolved, that is, the aromatic dihydroxy compound still remains in a solid state.
The solubility of bisphenol A in water is 1.2 mol/L at about 40.degree. C. in water of pH 11.2 (this is the best solubility) and about 0.05 mol/L in water of pH 10.5 according to Ind. Eng. Chem. Res., 30, 462 (1991). This indicates that an enormous amount of water is necessary to prepare a solution of bisphenol A at the pH value range of U.S. Pat. No. 3,989,672.
In addition, according to the process for the production disclosed in U.S. Pat. No. 3,989,672, unreacted aromatic dihydroxy compound is often mixed in the resulting aromatic polycarbonate, and the transparency of the resulting aromatic polycarbonate is not good.
This appears to be attributable to a phenomenon that at the beginning of the reaction some amount of an aromatic dihydroxy compound in a solid state is present and the strong agitation necessary for the interfacial reaction conditions causes scattering of the solid aromatic dihydroxy compound, for example, such solid aromatic dihydroxy compound attaches to the upper portion of the reactor and remains unreacted, and the unreacted aromatic dihydroxy compound is mixed into the organic solvent solution of the resulting aromatic polycarbonate.
That is, according to the above-mentioned polymerization method, it is often difficult to produce an aromatic polycarbonate completely free from unreacted aromatic dihydroxy compound.
It is known that unreacted aromatic dihydroxy compound present in an aromatic polycarbonate is a cause of color formation upon melt molding and lowering of molecular weight.
Further, it is known that an aromatic dihydroxy compound is liable to be colored in the presence of a base, and for purposes of preventing coloration it is desirable to dissolve an aromatic dihydroxy compound in a basic aqueous solution at a relatively low temperature (about 20.degree. C. or lower).
However, according to the production process of U.S. Pat. No. 3,989,672, the dissolution of the aromatic dihydroxy compound is effected after the reaction temperature has risen by exothermic reaction of the carbonyl halide compound with the aromatic dihydroxy compound unless any particular cooling means is used, and therefore, it is difficult to produce an aromatic polycarbonate of less coloration.
In addition, Japanese Patent Application Laid-open Nos. Hei 3 - 2216 and Hei 4 - 255717 disclose that an aromatic polycarbonate oligomer is produced by effecting a reaction with phosgene in the presence of a polymerization catalyst at a predetermined concentration. For producing an aromatic polycarbonate from the oligomer of the aromatic polycarbonate produced as mentioned above, a molecular weight controlling agent is added after the aromatic polycarbonate oligomer is produced, and there is a problem that the rate of end capping of the resulting aromatic polycarbonate is low.
In view of the foregoing, according to the methods of prior art, there have not yet been obtained aromatic polycarbonates having a properly controlled molecular weight, a high rate of end capping and less coloration.